The present invention relates to Surface Acoustical Wave (SAW) filters and, in particular, to a dispersive filter which employs a tapered transducer and a dispersive reflective array. The array has an electrode pattern which reduces insertion loss and electrode resistance which allows the electrodes to be connected in series electrically, in order to raise the acoustic impedance of the device.
Dispersive filters are useful in applications such as radar, or sonar, and in some nondestructive testing applications, where a transmitter emits a short pulse which is reflected from an object and returned to a receiver. Since the returning pulse is a function of the reflections from both the object under consideration and the surrounding environment and also noise signals, early systems of the pulse echo type used a high power, short duration pulse to obtain good range resolution and a high signal-to noise ratio. This imposed severe limitations on transmitter components.
Subsequent pulse echo ranging systems were developed which used a long duration signal of a relatively low peak power, and derived a narrow pulse signal through a matched filter at the receiver input. One type of such a matched filter is designed to provide a rising time delay versus frequency characteristic, whereby high frequencies are delayed for a greater period of time than are low frequencies. When a signal is applied to such a filter in which high frequencies occur at the beginning of the waveform, and lower frequencies occur toward the end of the waveform, compression of the signal will occur, and the duration of the compressed pulse will be less than the duration of the input waveform. This type of filter is commonly called an "up-chirp" filter because the time delay increases with frequency. Similar filters whereby the time delay decreases with frequency increase are known and are called "down-chirp" filters.
Dispersive transducers have been developed with an apodized structure wherein the input and output transducers are located along a line, and either signal expansion or signal compression can be achieved through proper design of the device. An example of a "two-bounce" SAW device which utilizes reflected acoustic waves is shown in U.S. Pat. No. 4,521,751 issued June 4, 1985 to Gerd Riha and Richard Veith, in which the input and output transducers are located side by side and two reflective arrays consisting of a series of angled reflective line elements or gratings are aligned so waves emitted from the input transducer bounce off of both of the reflective arrays back to the output transducer. One problem with a two-bounce reflective array is that a substantial portion of the delay path of a two-bounce device is in the reflective gratings, which results in a less efficient, or higher loss, device. In addition the two-bounce configuration results in a narrower allowable bandwidth due to a higher scattering loss into bulk modes, and bandwidth limitations of the transducers.
A Surface Acoustic Wave filter is shown in U.S. Pat. No. 3,753,164 issued Aug. 14, 1973 to Adrian J. DeVries. In this device the input transducer and output transducer were aligned with respect to an equally-spaced, multi-line, reflective array so that the angle between a normal to the input transducer and the reflector is equal to the angle between a normal to the output transducer and the reflector. This configuration was employed in combination with a separate dispersive element, which was located behind the reflective array, to minimize the response of the output transducer to bulk waves.
A dispersive SAW filter is also known in the prior art in which the input and the output transducers were aligned similar to the alignment of the DeVries device, but the reflective array had a nonlinear displacement function so that the elements of the array which are closer to the transducers were more closely spaced than those which were further removed from the transducers. The input and output transducers of this device, because of their constant periodicity, were responsive only to a relatively narrow bandwidth. Although the array was dispersive because its periodicity changed continuously along the array, only a small portion of the array was effective in reflecting usable signals, since other portions of the array were either too narrowly, or too widely, spaced to reflect the entire bandwidth coherently.
Another line of development, which is concerned with wide-band delay lines in which SAW filters using slanted finger transducers were developed, is described in "Wide Band Linear Phase SAW Filters Using Apodized Slanted Finger Transducers" by P. M. Naraine and C. K. Campbell in the 1983 IEEE Ultrasonic Symposium Proceedings, pages 113-116. The slanted finger interdigital transducer structure described in this article employed straight slanted fingers which emanated from a common focal point, in an effort to yield a flat amplitude response across a passband. Apodization of the device was derived from a computer optimization routine to compensate for the inherent negative amplitude slope of an unapodized slanted finger transducer, so that the external amplitude equalization circuits were not needed.
A subsequent article entitled "Improved Modeling of Wide-Band Linear Phase SAW Filters Using Transducers with Curved Fingers" by N. J. Slater and C. K. Campbell was published in the IEEE Transactions on Sonics and Ultrasonics, Vol. SU-31, No. 1, January 1984, pages 46-50. The authors of this work describe a wide band linear phase SAW filter in which slanted fingers, such as those shown in the Naraine and Campbell article have been curved to obtain a flatter frequency response for delay line applications.
The present invention involves the use of transducers with curved fingers which are, in particular, hyperbolically tapered in conjunction with a dispersive reflective array to provide an improved SAW dispersive filter. The transducers of the present invention also utilize configurations which reduce the insertion loss and allow for the acoustic impedance to be increased by a unique finger design.